Recently, there has been a proliferation in the field of wireless communications. Devices such as cordless and cellular telephones, pagers, wireless modems, wireless email devices, personal digital assistants (PDAs) with communication functions, and other portable electronic devices are becoming commonplace. To preserve the integrity of transmission to and from such devices, it is important to maintain a clear, strong radio signal.
A gain of an antenna of a portable electronic device, such as a pager or a cellular telephone be may be lowered by the presence of certain objects such as metallic objects and/or by ground plane conditions, i.e. the antenna may be detuned. When the antenna is detuned, the portable electronic device may have a shorter operating radius and may experience poor in-building performance or fringe performance, i.e. performance at the edge of a cell.
Embedded antennas are common in conventional portable electronic devices. A signal transmitted from or received by an antenna which is integrated into a portable electronic device may encounter several boundaries, including for example a printed circuit board, a battery, a display screen, a device housing, a device carrying case and any of a multitude of other elements or components associated with the device, in addition to a user's body. All such boundaries may influence the propagation of the signal and the surrounding impedance seen by the antenna.
Furthermore, when a portable electronic device is in operation, it may be turned in different directions and may not be optimally aligned to receive and/or transmit signals. At least a portion of signal power losses associated with antennas in portable electronic devices is due to signal reflection. Ideally, all of the signal power of a signal input to an antenna would be converted into an electromagnetic signal and radiated. Likewise, ideally all electromagnetic energy received at the antenna would be converted into an electrical signal and provided to receive electronics. However, in reality the characteristic impedance of the portable electronic device interacts with the characteristic impedance of the antenna and unless these impedances are equal, some signal reflection will likely occur.
Accordingly, some conventional portable electronic devices may include an impedance matching circuit between communication circuitry of the portable electronic device and an antenna. Impedance matching circuits typically include an LC circuit with inductance and capacitance elements connected in any one of a number of standard matching circuit topologies. However, some conventional impedance matching circuits may be configured during manufacture of the portable electronic device and may not provide for adjustments in the field. Thus, the matching circuit may not adapt to environmental conditions that may influence the impedance affecting the antenna. For example, over the air signals transmitted and received by an antenna of a portable electronic device may encounter such dielectric boundaries as the housing of device, printed circuit boards, electronic components in the housing, batteries for powering the device, display, input device and the body of a device user, all of which may influence the impedance seen by the antenna. Such impedances can be estimated, but are dependent upon the orientation of the device with respect to its surroundings. Thus, even the best estimates of impedance matching requirements may not remain accurate for all device operating conditions.
To address possible problems with matching circuits discussed above, adaptive antenna assemblies have been provided in some conventional portable electronic devices such that the antenna may adapt to environmental conditions. For example, adaptive antennas are discussed in U.S. Pat. No. 6,570,462 to Edmonson et al., U.S. Pat. No. 5,778,308 to Sroka et al. and U.S. Pat. No. 5,564,086 to Cygan et al. However, improved antenna matching circuits may be desirable.